Restored wetlands may never recover

UC BERKELEY (US) — Even after a century of restoration efforts, some wetlands are never able to return to their original natural state.

“Once you degrade a wetland, it doesn’t recover its normal assemblage of plants or its rich stores of organic soil carbon, which both affect natural cycles of water and nutrients, for many years,” says David Moreno-Mateos, a postdoctoral fellow at the University of California, Berkeley. “Even after 100 years, the restored wetland is still different from what was there before, and it may never recover.”

Published in the journal PLoS Biology, a new analysis calls into question a common strategy used by land developers: create a new wetland to replace a wetland that will be destroyed and the land put to other uses. At a time of accelerated climate change caused by increased carbon entering the atmosphere, carbon storage in wetlands is increasingly important, Moreno-Mateos says.

“Wetlands accumulate a lot of carbon, so when you dry up a wetland for agricultural use or to build houses, you are just pouring this carbon into the atmosphere,” he says. “If we keep degrading or destroying wetlands, for example through the use of mitigation banks, it is going to take centuries to recover the carbon we are losing.”

Wetlands tend to recover most slowly if they are in cold regions, if they are small, less than 100 contiguous hectares, or 250 acres, or if they are disconnected from the ebb and flood of tides or river flows.

“These context dependencies aren’t necessarily surprising, but this paper quantifies them in ways that could guide decisions about restoration, or about whether to damage wetlands in the first place,” says coauthor Mary Power, professor of integrative biology.

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But restored wetlands contain about 23 percent less carbon than untouched wetlands, and the variety of native plants is 26 percent lower, on average, even after 50 to 100 years of restoration. While restored wetlands may look superficially similar—and the animal and insect populations may actually be similar—the plants take much longer to return to normal and establish the carbon resources in the soil that make for a healthy ecosystem.

While several studies have shown that specific wetlands recover slowly, Moreno-Mateos believes the new study “might be proof that this is happening in most wetlands. To prevent this, preserve the wetland, don’t degrade the wetland.”

The new research included 124 wetland studies monitoring work at 621 wetlands around the world and comparing them with natural wetlands. Nearly 80 percent were in the United States and some were restored more than 100 years ago, reflecting a long-standing American interest in restoration and a common belief that it’s possible to essentially recreate destroyed wetlands. Half of all wetlands in North America, Europe, China, and Australia were lost during the 20th century.

Though on average, restored wetlands are 25 percent less productive than natural wetlands, variation exists. For example, wetlands in boreal and cold temperate forests tend to recover more slowly than do warm wetlands. One review of wetland restoration projects in New York state, for example, found that “after 55 years, barely 50 percent of the organic matter had accumulated on average in all these wetlands” compared to what was there before.

“Current thinking holds that many ecosystems just reach an alternative state that is different, and you never will recover the original,” says Moreno-Mateos, who plans in the future to explore whether the slower carbon accumulation is due to a slow recovery of the native plant community or invasion by non-native plants.

The work was supported by the Spanish Ministry for Innovation and Science, the Spanish Foundation for Science and Technology, and the National Center for Earth Surface Dynamics of the U.S. National Science Foundation Science and Technology Center.